feat(topics): add utility funcs to topic model

topics/models/topic.py

@@ -1,12 +1,32 @@
 from django.db import models
+import random
 from topics.models.pool import TopicPool
 
 
+class TopicManager(models.Manager):
+    def sample_topic_slugs(self, order, pool: str = None, limit=10) -> list[str]:
+        if pool:
+            topics = self.get_topic_slugs_by_pool(pool)
+        else:
+            topics = self.all().values_list('slug', flat=True)
+        if order == 'random':
+            return random.sample(list(topics), min(limit, len(topics)))
+        else:
+            raise Exception("Invalid order: '{}'".format(order))
+
+    def get_pools_by_topic_slug(self, topic_slug: str) -> list[str]:
+        return self.filter(topic_slug=topic_slug).values_list("pools__name", flat=True)
+
+    def get_topic_slugs_by_pool(self, pool: str) -> list[str]:
+        return self.filter(pools__name=pool).values_list("slug", flat=True)
+
+
 class Topic(models.Model):
     slug = models.CharField(max_length=255, unique=True)
     en_title = models.CharField(max_length=255, blank=True, default="")
     he_title = models.CharField(max_length=255, blank=True, default="")
-    pools = models.ManyToManyField(TopicPool, related_name="topics")
+    pools = models.ManyToManyField(TopicPool, related_name="topics", blank=True)
+    objects = TopicManager()
 
     def __str__(self):
         return f"Topic('{self.slug}')"